1. Field of the Invention
The present invention relates to a method of separating and recovering the useful elements of tungsten and titanium from a used DeNOx catalyst.
2. Description of Related Art
When a coal-fired boiler, a heavy oil-fired boiler, or a combustion furnace fitted to any of various chemical apparatus is operated, an exhaust gas containing nitrogen oxides (hereafter abbreviated as NOx) is discharged. Because NOx is an atmospheric pollutant that generates photochemical smog and acid rain, the NOx must be removed from the exhaust gas prior to discharge of the exhaust gas from the plant. One method that is used for removing NOx from the exhaust gas is the selective catalytic reduction method. The selective catalytic reduction method is a method that uses a reduction catalyst to react the NOx with ammonia, thereby decomposing and detoxifying the NOx. The selective catalytic reduction method is recognized as the most economic and effective method, and is widely used industrially.
FIG. 6 shows an example of the structure of a DeNOx apparatus that uses the selective catalytic reduction method. In FIG. 6, a combustion exhaust gas generated in a boiler 1 passes through a super heater 2 and an economizer 3, before reaching a flue 4 that guides the exhaust gas into a DeNOx reactor 6. An ammonia injector 5 is provided in the flue 4, and the ammonia gas that is required for the DeNOx reaction is injected into the flue 4 from the ammonia injector 5. The NOx within the combustion exhaust gas is decomposed into nitrogen and water during passage through a catalyst layer 7 disposed inside the DeNOx reactor 6. Subsequently, the combustion exhaust gas passes through an air heater 8, an electrical dust precipitator 9 and a combustion exhaust gas fan 10, before being discharged into the atmosphere from a chimney 11.
The catalyst layer 7 disposed inside the DeNOx reactor 6 is composed mainly of a gas parallel flow-type catalyst having a lattice-like or plate-like form. In a gas parallel flow-type catalyst, the combustion exhaust gas flows in parallel along the surface of the DeNOx catalyst. As a result, the dust and soot within the combustion exhaust gas has little chance to contact the surface of the DeNOx catalyst, thus offering the advantage that adhesion of the dust and soot to the catalyst surface is minimal. Accordingly, gas parallel flow-type catalysts are widely employed in DeNOx apparatus for coal-fired boilers and heavy oil-fired boilers and the like.
The DeNOx catalyst used in these types of DeNOx apparatus employs titanium oxide (TiO2) as the base material. Active components such as vanadium pentoxide (V2O5), tungsten oxide (WO3) and molybdenum oxide (MoO3) are supported on the base material.
The above DeNOx catalyst exhibits superior DeNOx performance over a wide temperature range. However, even though this superior performance is maintained during an initial period, the DeNOx performance gradually deteriorates when the catalyst is used for a long period. Examples of the causes of this deterioration in the DeNOx performance include (1) adhesion of dust and soot to the surface of the DeNOx catalyst, blocking holes through which the gas passes, (2) diffusion into the DeNOx catalyst of a poison component contained within dust and soot adhered to the surface of the catalyst, resulting in poisoning of the DeNOx catalyst, and (3) gasification of a substance contained within the fuel inside the furnace that results in the formation of a catalyst poison, which subsequently undergoes physical adsorption to the DeNOx catalyst or chemical reaction with a component of the catalyst, thus impairing the progress of the DeNOx reaction.
The deterioration in performance caused by adhesion of dust and soot to the surface of the DeNOx catalyst, as described above in (1) and (2), can be suppressed by providing a dust removal device at the combustion gas inlet side of the catalyst layer 7, thus reducing the amount of dust and soot reaching the catalyst layer 7.
However, in the case of poisoning of the DeNOx catalyst by a gaseous component, as described above in (3), there are currently no countermeasures available for preventing the poison component from penetrating into the catalyst layer 7. As a result, the durability of the DeNOx catalyst varies considerably depending on the types and amounts of poisonous substances contained within the fuel.
In a coal-fired boiler, coal is used as the fuel, but the quality of coal varies considerably depending on the area in which it was produced, and some coal contains a large amount of arsenic. This arsenic causes deterioration of the catalyst, and once the DeNOx catalyst has deteriorated, it is replaced. Currently, this used DeNOx catalyst is discarded, with no attempt made to recover the rare metals such as tungsten and titanium from the catalyst.
Examples of methods used for separating and recovering rare metals from ore and the like include the chlorination volatilization method disclosed in PTL 1 and PTL 2. The chlorination volatilization method is a method in which the separation target material (such as the ore) is heated in a chlorine atmosphere to convert the various components to chlorides, and the difference in boiling points of these chloride compounds is then used to volatilize and separate the compounds.
PTL 3 and PTL 4 disclose the addition of a chloride of an alkaline earth metal such as calcium chloride as a chlorine source to the raw material containing the heavy metal elements in the chlorination volatilization method. PTL 5 discloses a chlorination volatilization method in which a calcium compound is added to the raw material, a heat treatment is performed under an inert atmosphere, W, Nb, Ni and Co are then volatilized under a gas stream of chlorine to produce solid compounds having a high concentration of Ta, Cr and Ti, and these high-concentration solid compounds are subsequently mixed with solid carbon and heated in a chlorine atmosphere to volatilize Ta, Cr and Ti. In PTL 3 to PTL 5, Ca or the like is added to change the state of the compounds, thereby increasing the reactivity with chlorine.    PTL 1: Japanese Unexamined Patent Application, Publication No. 2009-132960 (claim 1, and paragraphs [0017] to [0019])    PTL 2: Japanese Unexamined Patent Application, Publication No. 2011-74408 (claim 1, and paragraphs [0028] to [0034])    PTL 3: Japanese Unexamined Patent Application, Publication No. 2007-268339 (paragraphs [0052] to [0057])    PTL 4: Japanese Unexamined Patent Application, Publication No. 2008-49204 (claims 1 and 7, and paragraphs [0043] to [0048])    PTL 5: Japanese Unexamined Patent Application, Publication No. 2008-222499 (claim 1, and paragraphs [0010] to [0011])